Fence tampering alarm system

ABSTRACT

The alarm system of this invention is intended primarily for use in conjunction with metallic fences and will detect the act of cutting the fence, climbing the fence, and certain forms of digging under the fence. The alarm system is useful in unattended applications, that is, visual monitoring of the system is unnecessary. The intrusion detection system of this invention embodies an external noise sensitive elongated transducer element which may be mechanically attached to a metallic fence along its length such that mechanical vibrations (noise) associated with the cutting of the fence may be conducted to the elongated transducer. The transducer is a two conductor device of coaxial design with a high impedance dielectric material, such as Teflon, separating the two conductors. The pulse signature output of the transducer, taken across the conductors, is processed in an electronic signal detection means to trigger an alarm.

Skujins, Jr.

0R iateassme United St:

i 1 FENCE TAMPERING ALARM SYSTEM [75] inventor: Romans Skujins, Jr.,Clifton, Va.

[73] Assignee: The United States of America as represented by theSecretary of the Army, Washington, DC.

22 Filed: Aug. 18,197]

21 Appl. No.: 172,912

[52] US. Cl. .L 340/15, 340/261 [51] Int. Cl. H04b 11/00 [58] Field ofSearch 340/261, 253, 254, 15

[56] References Cited UNITED STATES PATENTS 2,403,503 7/l946 Coulter.340/261 3,610,808 10/1971 Horwinskin". 340/261 2,345,771 4/1944 Reynolds340/261 Primary Examiner-Benjamin A. Borchelt Assistant ExaminerHt J.Tudor Attorney, Agent, or Firm-Edward J. Kelly; Herbert Berl; Glenn S.Ovrevik 1111 3,803,548 [455?] Apr. 9, i974 [57] ABSTRACT The alarmsystem of this invention is intended primarily for use in conjunctionwith metallic fences and will detect the act of cutting the fence,climbing the fence, and certain forms of digging under the fence. Thealarm system is useful in unattended applications, that is, visualmonitoring of the system is unnecessary. The intrusion detection systemof this invention embodies an externauqise s e n sitive elongatedtransducer element which may be mechanically attached to a metallicfence along its length such that n eghanicgl vibrations (noise)associated with the cutting of the fence be conducted to the elongatedtransducer. The

transducer is a two conductor device of coaxial design with a highimpedance dielectric material, such as Teflon, separating the twoconductors. The pulse signature output of the transducer, taken acrossthe conductors, is processed in an electronic signal detection means totrigger an alarm.

I 8 Claims, 6 Drawing Figures I -aiziifatfizaitziz DETECTOR 1 KTELECTRONlCS MODULE U DETECTOR POWER steam/mus PM I ATENTEDAPR 9 IBM sum1 [IF 3 P POWER PM TERMINATION DETECTOR MODULE DEVICE- 15 i 22; DETECTORELECTRONICS MODULE ELECTRONICS ALARM I TERMINATION DEVICE POWER PAKELECTRONICS MODULE 34 DETECTOR ALARM 1N VENTOR 8 V .lv rU S germ m dr oV r wlnu FW udel HE G PATENTEUAPR 9 m4 SHEET 2 BF 3 MmIIm2 j N55 Q SEE FESE PATENTED R 9 I974 SHEET 3 [IF 3 Fig. 6

INVENTOR Romans Skujms, Jr".

Harry M. Sara avifi Edward J. KelL Herbert Berl Glenn 6'. Ovrevik FENCETAMPERING ALARM SYSTEM BACKGROUND OF THE INVENTION This inventionrelates to devices for the detection of intruders entering an enclosedarea having a perimeter defined by a metallic fence structure. A widevariety of devices have been developed for such detection purposesincluding various crystal and electromagnetic transducer devicesadaptedto be rigidly attached to the fence. In one systemembodyingindividual detectors, described in U.S. Patent No. 2,345,771,'themetallic fence is attached to metallic fence posts and the transducerssense vibrations transmitted to the posts. In another system, describedin U.S. Patent No. 3,487,396, a pluralityof sensors are attached tofence elements in both rigid and semi-rigid fashion and electroniccircuitry is provided to discriminate between the outputs ,of variouslydisposed sensors. In an alternate assembly which is more analagous tothe present invention, reference U.S. Patent No. 2,403,503, a taut pianowire stringer is attached to the fence and the transducers are attachedto the taut wire.

It will be appreciated that highly sensitive systems have been developedand that as the sensitivity of most systems is increased, the frequencyof false alarm occurance also increases. Likewise, it will be recognizedthat the complexity and the expense of alarm systems which utilize aplurality of discrete point sensors generally increases with increasedsensitivity.

It is an objectof this invention to provide a relatively simple and lowcost alarm system with minimum sensor associated electronics, which isreadily adaptable to existing metallic fences.

It is another object of this invention to provide a uniformly sensitivesystem for sensing sonic signal spectra generated. by acts such ascutting of the fence.

It is a further object of this invention to provide a highly sensitive.alarm system which permits discrimination between natural and man-madefalse alarm stimuli.

It is also an object of the invention to provide an alarm system whichdiscriminates against acoustic background noise and noise of seismicorigin.

SUMMARY OF THE INVENTION The fence tampering alarm system of. thisinvention embodies a uniformly sensitive transducer cable which servesto minimize the distance that the sonic impulse signal (generated bycutting action or the like) has to travel prior totranslation into anelectrical signal. In this manner, the characteristics of the cuttingsignatures are preserved: sufficiently to permit discrimination betweennatural and deliberate man-made alarm stimuli with a minimum signalprocessing effort. The shielded aspect of the cable transducerconstruction minimizes the effect. of electromagnetic noise and the lowair-to-metal coupling coefficient involved, minimize the effect ofacoustic background noise.

The alarm system electronics are primarily designed to detect relativelyhigh frequency signal pulses in the 500 Hz to 6,000 Hz frequency band.Such pulses are produced. during the act of cutting fence wires. Thehigher frequency pulse signal is generated as a result of asuddenrelease of: stresses in the wire at the moment thecut: iscompleted. The higher frequency pulse signal is especially pronounced inchain link fence applications. It has been found that this higherfrequency pulse signal is attenuated in proportion to the distance thesignal travels through the fence wire network. In accordance with theinvention, the uniformly sensitive transducer cable is rigidly attached,at spaced intervals, to the fence wire network with each end of thecable transducer terminated in an appropriate termination means tostabilize sensitivity. The disposition of attachments, of course,determines the distance the high frequency must travel through the fenceto reach the cable transducer. A signal processing electronics moduleresponsive to pulse energy is attached to one end of the transducercable.

A better understanding of the invention will be gained by a review ofseveral embodiments of the invention for which reference is had to thedescription which follows and the illustrations depicted in the attacheddrawings wherein:

FIG. 1 depicts a single ended alarm system designed to monitor a fencesegment.

FIG. 2 depicts a differential signal embodiment of the alarm systemdesigned to monitor a large fence segment.

FIG. 3 is a block diagram of a preferred embodiment of the electronicsmodule for use with the system of FIG. 2.

FIG. 4 is a cutaway view of a first form of uniformly sensitivetransducer cable which may be utilized in this invention.

FIG. 5 is a cutaway view of a second form of uniformly sensitivetransducer cable which may be utilized in this invention.

FIG. 6 is a graphical presentationv of various pulse informationdeveloped in the translation of noise to an electrical signal which isintroduced to the electronics module for discrimination processing inthe alarm system of this invention.

DETAILED DESCRIPTION OF THE INVENTION FIG. 1 depicts a chain link fencesegment lll monitored in accordance with the system of this inventionwhich embodies a single, uniformly sensitive, transducer cable 12 whichis mechanically attached to the fence wire by ties 113 at spacedintervals. The cable 112 is terminated at the end remote from thedetector electronics module 14 by an appropriate impedance terminationmeans 15. The impedance termination means enables a tamper monitoring ofthe transducer cable as will be described hereinafter. A power pak I6 isconnected to the detector electronics module as an energy power sourcetherefor.

While the cable 12 is shown in an earth buried application, it will beappreciated that the cable may be attached to the fence above ground, ifdesired. Indeed, it has been found that the uniformly sensitive cablefence tampering system of this invention may be utilized advantageouslyirrespective of the fence environment, for example, the system may beused in above ground applications and in underwater metal fence barrierapplications as well.

FIG. 1 also depicts a second fence tampering alarm system of the pointsensor variety comprising a plurality of individual sensors 21, forexample, xtal devices as shown in U.S. Patent No. 3,478,343, and adetector electronics module 22 interconnected by wiring, indicated at23. It will be recognized that the second fence tampering alarm systemis entirely independent of the basic uniformly sensitive transducercable system and is not essential to the operation of the basic cablesystem. It has been found, however, that the two types of fencetampering alarm systems are compatible and may be utilized together asshown to optimize false alarm rejection due to noise sources other thanan intrusion.

I embodiment serve to preserve the broad band characteristics of eachtransducer cable and to impedance match the two transducer cables, and apower pak 16 is connected to the detector electronics module 34 as anenergy power source therefor.

It will be noted that FIG. 2 depicts an aboveground connection of thetransducer cable to the fence. As in the embodiment of FIG. l,theprecise disposition of the uniform impedance transducer cable is notcritical to the invention. Likewise, the number of rigid connections ofthe transducer cable to the metallic fence may be varied to meetpractical requirements. That is, the ties 13 may be applied to rigidlyfasten the cable to the concertina fencing in the embodiment of FIG. 2wherever a cable-fence wire crossing occurs. It will be recognized thatthe present invention is readily adaptable to a variety of differentmetallic fences and that fence supports are not essential to theoperation of the device. In other words, the transducer cable may be.attached to the fence in a taut fashion or merely draped on the fence asshown in FIG. 2.

FIG. 3 shown a typical embodiment of the detector electronics module 34in a block diagram schematic form. In the illustration of the typicalembodiment, the electronic detection circuitry is connected to twouniformly sensitive cables 32 and 33 of the coaxial variety and eachcoaxial cable is terminated by a resistive impedance termination means15. Obviously, any appropriate type of simple or complex impedancetermination means may be substituted for the resistive device shown.

The input of the exemplary detector electronic module may be atransformer, as shown at 41, with a center tapped input winding. As theouter conductor of each of the transducer cables 32 and 33 is grounded,the center tap of the input winding may be DC grounded or AC groundedthrough a capacitor 42, as shown. For reasons which will become apparenthereinafter, the capacitor 42 and resistor 43 may be connected in seriesas a voltage divider across the battery 44.

The output winding of the transformer 41 is connected to audio amplifier45 via switching means 46 and either high band-pass filter 47 or lowband-pass filter 48. Typically, the filters 47 and 48 might havebandwidths 3,0006,000 I-Iz (i3db) and 500l,500 Hz (i3db) respectively,with each having a band-pass selectivity skirt at a rate of attenuationof l8db/octave beyond its specified frequency range.

The signal input to the audio amplifier 45 which passes the filter 47 or48 has an envelope, as shown, and in processing through the amplifier45, the ampliclip high amplitude short duration noise spikes such thatthe output thereof has a waveform envelope, as shown, of predeterminedmaximum amplitude and significant duration.

The output of the limiter 49 is applied to a rectifier circuitry 51which serves as an envelope detector to produce a square wave pulseoutput, as shown, of significant amplitude and duration. The output ofrectifier 51 is applied to an integrator 52, which may be a simple RCnetwork, which produces a pulse with a constant rise time and anamplitude proportional to the amplitude and duration of the input signalthereto.

Thereupon, the pulse output of the integrator 52 is applied to a leveldetector 53 which produces an alarm pulse in the event the amplitude ofthe pulse output of the integrator exceeds a predetermined level. Itwill be appreciated that the level detector 53 may be a signalcomparator means and may include a threshold reference signal source,not shown, for comparison purposes.

The alarm pulse may be utilized in any conventional manner to energizethe alarm 54 which may be disposed in a remote location, if desired.Typically, the alarm pulse is applied through an intermediate alarminterface 55 which also affords means 56 for detecting any battery orcase tampering which could inactivate the system and also may include atest command circuitry 59 for periodic checking of the operational stateof the detector electronics module. It will be recognized that any formof battery or case tampering signal in voltage occurs at the centertapconnection and this change in voltage may actuate the tamperingdetection means 57, as prescribed.

For optimum performance, the detector electronics module embodiment ofFIG. 3 should be. relatively insensitive to variations in ambienttemperature and power supply voltage deviations. It has been found thatthe effect of such deviations can be minimized by the selective use oftherrnistors and/or other compensating circuit techniques. It will berecognized that recovery time, that is, the time required to reset todetect a second intruder should be short, i.e., within several secondsafter a saturating signal. The fence tampering alarm system of thisinvention is intended for long term, maintenance free operation, forexample, one year, and it will be recognized that a low current drainrequirement which extends battery life is essential.

While an exemplary embodiment of the detector electronics has been shownin FIG. 3, it will be appreciated that various other detector circuitrywhich will accept a damped oscillation signal and reject other spurioussignals, such as high amplitude, short duration noise spikes, may besubstituted, if desired. In other words, the detector electronics shouldaccept the signal produced by a cutting action and produce an alarm butshould not be responsive to such action, for example,

as mechanical impact from dropping a regulation tennis ball on thedetector electronics module from a height of one meter while the moduleis resting on a two inch thick moist cellulose sponge, nor responsive toplacement within a Helmholtz coil test unit with 60 cycle currentapplied to coil unit.

Moreover, it will be seen that the switching means 46 is not essentialto the preferred embodiment of FIG. 3, and that if continuous monitoringat both frequency levels is desired, each of the filters 47 and 48 maybe connected to its respective transformer and transducer cable pairassembly with the output of the filters connected to the input of theaudio amplifier 45 through a conventional OR circuit, not shown.

The uniformly sensitive cable transducer shown in FIG. 4 is aconventional coaxial cable having a braided sleeve type outer conductor61 covered by a weather resistant plastic sheath 62 which may be aplastic material, polyvinylichloride or the like. The center conductor63 which may be a stranded wire, as shown, is separated from the outerconductor 61 by a high impedance dielectric 64. The dielectric 64, inmost commercially available coaxial cable, is of the Teflon variety(Teflon is a trademark held by E. I. duPont de Nemeurs & Co. foridentification of tetrafluoroethylene material.) The invention is not,of course, restricted to the use of transducer cable with thiscommercially available dielectric. The dielectric 64 may be formed onthe center conductor by any conventional method such as extrusion,wrapping, coating or the like.

It has been found that any commercially available coaxial cable may beutilized as the uniformly sensitive cable transducer in the system ofthis invention. Some coaxial cable is more noise sensitive than othercoaxial cable and for purposes of the present invention, the more noisycoaxial cable is preferred. The noise sensitivity of any particularcoaxial cable is dependent generally on the method of manufacture of thecable. Most manufacturers attempt, with varying degree of success, tomanufacture by methods which minimize the noise sensitivitycharacteristic. It has been discovered that the noise sensitivity ofconventionally manufactured coaxial cable can be enhanced for thepurpose of the system of this invention by subsequent cable processing.A useful method for subsequently processing the coaxial cable isdescribed and claimed in the copending patent application, Ser. No.111,291 which was filed Feb. 1, 1971 in behalf of Charles F. Burney, etal. The invention described and claimed in the aforesaid copendingpatent application was made under a Department of the Army contract.

The noise sensitivity characteristic of coaxial cable has beentheoretically explored and appears to be either magnetostrictive ortriboelectric in nature. Irrespective of the phenomenon involvedhowever, it has been found that acoustic pressure incident upon theuniformly sensitive cable transducer at its points of rigid connectionto the metallic fence produce a physical disturbance at such pointswhich excites the transducer cable and thus generates a significantvoltage deviation across the transformer connected end of the transducercable. It will be appreciated that the uniformly sensitive transducercable with a resistor termination is, in effect, a lightly chargedtransmission line which may be represented as a resistive impedance inparallel with a plurality of capacitors in parallel connection. When theacoustic pressure deforms the cable at the rigid connection points, theheretofore relatively quiescent transmission line is activated andcurrent flow within the lightly charged transducer cable produces anoutput voltage across its transformer connected terminals. Enhancementofthe noise sensitivity, in essence, involved increased development ofan electrically charged condition in the dielectric separating thecenter and outer conductors. It will be appreciated that for use as atransducer in the present fence tampering application, the chargedcondition must be maintained over an extended period of time. It hasbeen found that a wide variety of coaxial cable structures may beadapted for use in the system of this invention and that the particulardielectric utilized determines the degree and nature of the energizationrequired to produce a charged condition which will be maintained in thedielectric over an extended period of time.

For example, FIG. 5 depicts a commercially available flexible delay lineof the coaxial variety which has been successfully employed in thesystem of this invention. In FIG. 5, the center conductor 71 of thecoaxial cable structure is would in helical form with minimum pitchabout a flexible powdered iron core 72 and the outer conductor 73consists of parallel wires wound about the dielectric 74 in spiral formwith a high pitch to the spiral winding. The dielectric 74 may be, forexample, a polyethylene or other low loss dielectric which is applied inthe form of overlapping tapes, as shown. A strong, weather resistantpolyvinlychloride jacket 74 is extruded over the outer conductor 75. Theparticularly described cable is commercially identified as a DELAX delayline and are produced by Kabel-und Metallwerke, 3000 Hannover, Postfach260, a German manufacturer. DELAX cables are distributed-parameter delayelements and as such do not exhibit a cut off frequency typical of theconventional lumped parameter delay line design. Rather, the attenuationincreases gradually with increasing frequency. It has been found thatthe impedance and bandwidth characteristics of this type of coaxialcable are suitable for use in applications involving cable transducerlengths of 50 feet. In installations utilizing DELAX type III-14000cable, tests have been conducted in conjunction with a four foot highchain link fence which have demonstrated that the system will detect atleast percent of all fence cuts with no false alarms in a high roadnoise and 40 mph wind environment. Such results were obtained withconventionally manufactured DELAX type HH4000 cable without additionalprocessing of the cable to enhance the energization of the dielectric.

It will be appreciated that in both the braided outer conductorconstruction shown in FIG. 4 and the parallel wire spiral outerconductor construction shown in FIG. 5, the electrically energizeddielectric of the transducer cable is omnidirectionally shielded fromextraneous electrical noise. Thus, such extraneous noise does notphysically disturb the internal structure. It has been found that theeffect of substitution of one form of cen ter conductor for another,stranded, solid, helix wound or the like, is minimal.

FIG. 6 illustrates noise pulse signals typically encountered which mustbe discriminated and excluded in normal operation of the system of thisinvention.

The high level, short duration pulse indicated at 81 in FIG. 6 isgenerated by electrical transients in the vicinity of the fence.Integration of the pulse 81 by the system of this invention produces anoutput pulse of constant rise time, indicated at 84, which isinsufficient in magnitude to trigger the alarm.

The initially high level, damped oscillation pulse indicated at 82 inFIG. 6 is generated by a strong cut of the wire fencing material.Integration of the pulse 82 by the system of this invention produces anoutput pulse of constant rise time, indicated at 85, which is ofsufficient magnitude to trigger the alarm. Likewise, the weak cut,damped oscillation pulse, indicated at 83, produces, when integrated bythe system of this invention, an output pulse of constant rise time,indicated at 86, which is of sufficient magnitude to trigger the alarm.

It will be recognized that in adapting the system of this invention toan existing fence installation, the cable transducer may be rigidlyattached to the fence by a variety of connection means. For example, thecable may be attached by wire hog rings using pliers or by a simple wiretwist which enables quick attachment of the cable transducer to themetal fencing at selected, spaced, intervals.

While each of the coaxial cable transducers described herein may beconstructed by attachment of suitable termination means to coaxial cablewhich is readily available in the commerical market, it will beappreciated that the invention is not restricted to the use ofconventional coaxial cable. For example, the cross-section configurationmay be different and/or the cable may be adapted to serve additionalpurposes, if desired. Likewise, the electronics employed to discriminatebetween wanted and unwanted signals may be modified by simplification orrefinement without departure from the purview of this disclosure. Forexample, other analog or digitaltechniques may be utilized instead ofthe disclosed RC integrator. In its simplest form, of course, thetransducer cable may-be directly connected to a single stage voltagelevel sensitive device which is adapted to trigger an alarm. I claim: 1.An intrusion detection system for use in conjunction with a metallicfenced enclosure comprising:

at least one elongated transducer cable of the coaxial cable varietyhaving an inner conductor and an outer conductor separated by adielectric material wherein said dielectric material is characterized bya relatively high noise sensitivity in response to low level mechanicalvibration disturbance of said outer conductor thereof; means formechanically attaching said elongated transducer cable means to themetallic fence at selected space intervals along the fenced perimeter ofsaid enclosure such that relatively low level mechanicalvibratimliillmfildamql llic rWEFe condiicfed to said outer conducto rofs'aid transducer cable means; alarm means; electronic signal detectionmeans electrically interconnecting said transducer cable means and saidalarm r ga rgs, said signal detection means having an input meanselectrically connected across said dielectric material, said signaldetection means adapted to detect input pulse signals and to activatesaid alarm means in response to selected input pulse signals producedacross said dielectric material by a cutting of the metallic fence, saidselected pulse signal signals having at least a selected amplitude andat least a selected pulse duration.

2. An intrusion detection system as defined in claim 1 wherein saiddielectric material has an inherent static electric charged condition ofsufficient magnitude to produce a measurable voltage output in responseto deviations in said charged condition;

said outer conductor of said cable means being a substantiallycontinuous metallic surface which shields said dielectric material from'external electrical field effects.

3. An intrusion detection system as defined in claim 2 wherein a cuttingof said metallic fence produces a pulse output of significant amplitudeand duration and said electronic signal detection means includesintegrator means which produces an output of sufficient magnitude toactivate said alarm means in response to said cutting of the metallicfence.

4. An intrusion detection system as defined in claim 3 wherein impedancemeans interconnect said innner and outer conductors at each end of eachtransducer cable means, one of said impedance termination meansassociated with each transducer cable means is an inductive couplingmeans and said electronic signal detection means interconnect saidinductive coupling means and said alarm means.

5. An intrusion detection system as defined in claim 4 wherein saidelectronic signal detection means includes band-pass filter meansadapted to pass frequencies within a selected band.

6. An intrusion detection system asdefined in claim 5 wherein saidselected band of frequencies is in the 3,000 to 6,000 Hz range.

7. An intrusion detection means defined in claim 6 wherein two balancedtransducer cable means are rigidly attachedto the metallic fence, saidinductive coupling means is a transformer means with a center tappedinput winding, each of said transducer cable means is connected across arespective half of the center tapped input winding of said transformerand the input of said electronic signal detection means is connectedacross an output winding of said transformer means.

8. An intrusion detection means as defined in claim 7 wherein means areprovided for detection of system tampering, the last said means includean energized DC voltage divider network which incorporates eachtransducer cable means and its respective impedance termination means,and means for monitoring the DC voltage level at a selected point onsaid voltage divider network such that DC voltage deviations activatesaid alarm means.

1. An intrusion detection system for use in conjunction with a metallicfenced enclosure comprising: at least one elongated transducer cable ofthe coaxial cable variety having an inner conductor and an outerconductor separated by a dielectric material wherein said dielectricmaterial is characterized by a relatively high noise sensitivity inresponse to low level mechanical vibration disturbance of said outerconductor thereof; means for mechanically attaching said elongatedtransducer cable means to the metallic fence at selected space intervalsalong the fenced perimeter of said enclosure such that relatively lowlevel mechanical vibrations in said metallic fence are conducted to saidouter conductor of said transducer cable means; alarm means; electronicsignal detection means electrically interconnecting said transducercable means and said alarm means, said signal detection means having aninput means electrically connected across said dielectric material, saidsignal detection means adapted to detect input pulse signals and toactivate said alarm means in response to selected input pulse signalsproduced across said dielectric material by a cutting of the metallicfence, said selected pulse signal signals having at least a selectedamplitude and at least a selected pulse duration.
 2. An intrusiondetection system as defined in claim 1 wherein said dielectric materialhas an inherent static electric charged condition of sufficientmagnitude to produce a measurable voltage output in response todeviations in said charged condition; said outer conductor of said cablemeans being a substantially continuous metallic surface which shieldssaid dielectric material from external electrical field effects.
 3. Anintrusion detection system as defined in claim 2 wherein a cutting ofsaid metallic fence produces a pulse output of significant amplitude andduration and said electronic signal detection means includes integratormeans which produces an output of sufficient magnitude to activate saidalarm means in response to said cutting of the metallic fence.
 4. Anintrusion detection system as defined in claim 3 wherein impedance meansinterconnect said innner and outer conductors at each end of eachtransducer cable means, one of said impedance termination meansassociated with each transducer cable means is an inductive couplingmeans and said electronic signal detection means interconnect saidinductive coupling means and said alarm means.
 5. An intrusion detectionsystem as defined in claim 4 wherein said electronic signal detectionmeans includes band-pass filter means adapted to pass frequencies withina selected band.
 6. An intrusion detection system as defined in claim 5wherein said selected band of frequencies is in the 3,000 to 6,000 Hzrange.
 7. An intrusion detection means defined in claim 6 wherein twobalanced transducer cable means are rigidly attached to the metallicfence, said inductive coupling means is a transformer means with acenter tapped input winding, each of said transducer cable means isconnected across a respective half of the center tapped input winding ofsaid transformer and the input of said electronic signal detection meansis connected across an output winding of said transformer means.
 8. Anintrusion detection means as defined in claim 7 wherein means areprovided for detection of system tampering, the last said means includean energized DC voltage divider network which incorporates eachtransducer cable means and its respective impedance termination means,and means for monitoring the DC voltage level at a selected point onsaid voltage divider network such that DC voltage deviations activatesaid alarm means.